1. Field of the Invention
This invention relates to an exhaust gas purification device for an internal combustion engine, configured to inject an additive to be supplied to a catalyst.
2. Description of the Related Art
In order to purify exhaust gas of diesel engine automobiles (vehicles), an exhaust gas purification device using an NOx trap catalyst, an NOx selective reduction catalyst, a particulate filter (diesel particulate filter) and/or the like in combination is used to prevent NOx (nitrogen oxides) and PM (particulate matter) in exhaust gas of the diesel engine from being emitted into the atmosphere.
For such exhaust gas purification devices, increasingly being adopted is a configuration in which a catalyst called a pre-stage catalyst, such as an oxidation catalyst or an NOx reduction catalyst (NOx trap catalyst or NOx selective reduction catalyst), is disposed in an exhaust passage for conveying exhaust gas discharged from the engine to the outside, and a fuel addition valve (reducing-agent addition valve) for injecting fuel as an additive required for reaction promoted by the catalyst is disposed upstream of the catalyst, for example, the oxidation catalyst.
In such exhaust gas purification devices, in order to enhance the purification efficiency in the cold state of the engine, the pre-stage catalyst is disposed near the exhaust side of the engine.
The space in the engine room is, however, limited. Thus, as shown in a patent gazette (Japanese Patent Laid Open No. 2005-127260), for example, there is a tendency to use an exhaust passage including a bend, for example an L-shaped bend to allow a pre-stage catalyst to be disposed directly downstream of the bend, and inject fuel from the outside of the bend toward an inlet end face of the catalyst, disposed directly downstream of the bend.
In this configuration, however, the flow of fuel injected from the outside of the bend toward the catalyst merges into exhaust gas passing through the bend, therefore curving, so that the fuel flow is liable to be constantly pushed from the inside to the outside of the bend by the exhaust gas passing through the bend.
Thus, in high-load operation of the engine with an increased flow volume and velocity of exhaust gas, the exhaust gas pushes the injected fuel flow from the inside of the bend with an increased force, so that the fuel flow deviates from a predetermined position on the inlet end face of the catalyst, for example from the center toward the side of the catalyst corresponding to the outside of the bend. In low-load operation of the engine with a decreased flow volume and velocity of exhaust gas, in contrast, the exhaust gas pushes the injected fuel flow with a decreased force, so that the fuel flow deviates toward the opposite side of the catalyst. Such deviation of the fuel flow directly reflects the operating state of the engine and is liable to become excessively great.
This leads to the problem that the fuel required for reaction fails to be supplied to the pre-stage catalyst in a desired direction, so that the catalytic converter using the pre-stage catalyst fails to show satisfactory performance.